1. Field of the Invention
The present invention relates to a color cathoderay tube and, more particularly, to a color cathode-ray tube which has an improved deflection unit in which three electron beams are accurately converged on a screen, and neither a negative cross pattern nor a positive cross pattern will appear on any part of the screen.
2. Description of the Related Art
Of color cathode-ray tubes having a shadow mask, the most prevalent one is the type comprising an in-line electron gun assembly having three electron guns and a shadow mask having slot-like apertures. This type of color cathode-ray tube has a deflection yoke designed to generate such horizontal and vertical deflection magnetic fields that the red, green and blue electron beams pass through an aperture at the same time. These deflection magnetic fields are known, as is disclosed in, for example, Japanese Patent Disclosure No. 57-1857. More specifically, the horizontal deflection magnetic field is shaped like a pin-cushion as is shown in FIG. 1A, and the vertical deflection magnetic field is shaped like a barrel as is illustrated in FIG. 1B. When the cathode-ray tube has a relatively small deflection angle, these magnetic fields deflect the three electron beams correctly, thus achieving good beam-convergence on any part of the screen. Therefore, the tube need not have a device for correcting the beam-convergency. When the cathoderay tube has a great deflection angle, e.g., 100.degree. or 110.degree., a convergence error occurs which is generally called "crossing of vertical lines,". It is regarded as impossible to correct this convergence error even if the horizontal and vertical deflection magnetic fields are changed in characteristic, as is disclosed in Iwasaki et al. "SST Deflection Yoke for Use in an In-Line High-Precision Color" in the technical report of Journal of Television Society of Japan, ED619, 1982. More precisely, when a saddle-and-toroidal deflection yoke is incorporated in a color cathode-ray tube of a great deflection angle, positive anisotropic astigmatism occurs, inevitably forming a positive cross pattern (FIG. 2A) on the screen. When a saddle-and-saddle deflection yoke is incorporated in a color cathode-ray tube of a large deflection angle, negative anisotropic astigmatism occurs, inevitably forming a negative cross pattern (FIG. 2B) on the screen. Either anisotropic astigmatism is mainly attributed to the fact that the centers of deflection of the magnetic fields, which have been generated by the horizontal and vertical deflection coils, respectively, take different positions.
It is generally considered the most desirable that when a saddle-and-toroidal deflection yoke is used, the saddle coil for deflecting the electron beams horizontally have a center of deflection located closer to the electric guns than the center of deflection of the toroidal coil for deflecting the beams vertically. If the centers of deflections of both coils take the same position, a convergence error, generally known as a positive cross pattern (FIG. 2A), will be made. The positive cross pattern is corrected into such a neutral one as is shown in FIG. 2C when the center of deflection of the saddle coil is deviated toward the electron guns from the center of deflection of the toroidal coil. When the center of deflection of the saddle coil is further deviated toward the electron guns, however, a negative cross pattern will appear on the screen as is shown in FIG. 2B. Therefore, it is generally considered possible to change a positive or negative cross pattern to a neutral cross pattern by adjusting the positional relation between the horizontal deflection coil and the vertical deflection coil.
Indeed, neither a positive cross pattern nor a negative cross pattern appear on the horizontal axis, at point P0 in the region near this axis, or at points P2 in the corner regions when the positional relation of the horizontal deflection coil and the vertical deflection coil is properly adjusted. However, a positive cross pattern and a negative cross pattern are formed at other points, such as point P1 between the horizontal axis and any corner region, or point P3 located at the outer edge of any corner, even if the positional relation of the coils is adjusted. Besides, the cross patterns appearing at point P1 and the outer edge of the corner are inverted. This inversion of cross patterns is prominent in proportion to the deflection angle or the size of the screen. Therefore, in the conventional color cathode-ray, a magnetic plate or the like is attached to the deflection yoke in order to reduce the change of forming positive and negative cross patterns. The use of the magnetic plate, however, does not reduce this chance sufficiently.